The Zune sounds much better than the iPod in terms of quality? They should sound very similar, given the same set of headphones and the same approximate volume level.

You keep talking about sound quality, but aside from the problem with low volume (which is a different measure from quality), you're not giving any specifics.

Depending on the actual quality problems you're having, I wonder if your iPod is perhaps a defective one? Or maybe the iPod is fine and there's something wrong with the files you've put on it?

Because every iPod I've seen sounds just fine at any volume level. The earbuds they ship with the thing suck, but if I put a decent set of earbuds in place, it sounds great. And you've already tried yours with different sets of headphones, so that's not the issue.

First, when you are judging quality, tell us about the song files you're using? Where did they come from, did you rip them yourself in iTunes, what was iTunes ripping quality settings configured as, etc?

Next, when you say bad quality, be specific. Do you mean low volume? Do you mean distortion? Do you mean bad frequency response? Bad stereo separation? If you mean bad frequency response, precisely which frequencies are we talking about?

Oh, another thing I thought of. Sometimes if the headphone jack is somehow not letting you plug the headphones in properly (like the problem with recessed headphone jack on the first gen iphones), then the contacts on the headphone connector might get summed mono or you might get only the left or the right audio output played into both ears. That could definitely be interpreted as bad sound quality.

In any case, you should narrow down the problem, because volume does not equal sound quality, and if you're actually having an issue with sound quality, it might indicate a defective unit.